The present invention relates to a screw press for performing filtering and dewatering while causing an outer tube in a condensing zone to rotate, and more particularly, to a screw press capable of preventing screen clogging, even with a sludge of low concentration having a water load higher than a solids load.
In the past, a high polymer coagulant has been added to an organic sludge from sewerage, excrement, or waste water from food product processing so as to cause the formation of flock. For organic sludge having a low concentration, the method of using a centrifugal condenser or a floatation condenser for condensing, and performing filtering and dewatering was adopted, these condensing apparatuses raising the concentration of sludge, for example, from 1% to 4 to 5%, the sludge of this concentration being filtered and dewatered by a screw press. Those apparatuses all occupy a large installation area, and involve a high cost for drive power and high polymer coagulant, in addition to being troublesome to maintain and control. Although it is not a condensing apparatus, there is a known screw press as an apparatus for filtering and dewatering in the Japanese Patent Publication No. 3-78123, in which a filtering tube is divided at its middle into a sludge intake side and a discharge side, having a narrowed pitch for a screw vane at the sludge intake side, and the filtering tube at the sludge intake side is rotated at a speed slower than the rotational speed of a feed screw to thereby filter and dewater the sludge, while injecting a high-pressure fluid from outside the tube, thereby cleansing a caked layer attached to minute holes in the tube, so that a continuous filtering and dewatering effect is maintained.
Because the concentration of the sludge at the sludge intake side of the screw press is low, at an outer tube screen in the condensing zone the water load is greater than the solids load, making it necessary to discharge a large amount of water at the outer tube screen. The screw press operates at a low speed, and in a small screw press having an outer tube with a diameter of 200 mm, the rotational speed is as slow as 1 rpm to 1.5 rpm, and in a screw press having an outer tube with a diameter of 800 mm, the rotational speed is 0.03 rpm to 0.14 rpm. In currently used screw presses, the rotational speed of large-diameter screw presses is from {fraction (1/30)} to {fraction (1/10)} that of a small-diameter screw press, and the effectiveness of the scraping of the inner peripheral surface of the outer tube by the screw vane in preventing clogging in a large-diameter screw press is no more than {fraction (1/10)} that of a small-diameter screw press.
In particular, a high polymer thin film is formed by the high polymer coagulant, and in the condensing zone residue tends to become attached to the screen, thereby causing clogging, although in a screw press in which the filtering and dewatering tube at the sludge intake end is forcibly rotated, it is possible to achieve the effect of preventing screen clogging by injecting cleaning water continuously as the outer tube is rotated. However, the injected water passes through minute holes in the screen and enters within the outer tube, thereby diluting the concentrated sludge, thereby risking a halving of the effectiveness in concentration of the sludge. Additionally, because cleaning is performed continuously, a large amount of cleaning water is required.
Accordingly, it is an object of the present invention to provide a screw press, applying prior art for filtering and dewatering while causing a filtering tube to rotate at the sludge intake end of the screw press, with the sludge intake side of the filtering tube serving as a condensing zone, preventing clogging of an outer tube screen and enabling concentration, filtering, and dewatering of even an organic sludge that has been coagulated by a high polymer coagulant, and which has a low concentration.
The essence of the present invention is a screw press in which an outer tube, over which a screen is tensioned, is divided into a freely rotatable condensing zone at a front half thereof, and a filtering and dewatering zone at a rear half thereof, a freely rotatable screw shaft is provided within the divided outer tube, filtered fluid is separated from sludge supplied to a starting end of the condensing zone by the screen of the outer tube, and a cake is removed at a discharge port at a rear end of the outer tube of the filtering and dewatering zone. In this screw press, a sludge supply path is provided in the screw shaft, a supply hole of this supply path is formed at the starting end of the outer tube of the condensing zone, and a scraper extending on the outer periphery of the screw vane from the starting end to the ending end, this scraper being caused to make pressured sliding contact with the inner peripheral surface of the outer tube, so that filtering and dewatering are performed as the outer tube of the condensing zone and the screw shaft are rotated.
By forming the sludge supply hole in the screw shaft, sludge is supplied without being affected by the rotation of the screw vane, without flock that is formed by coagulating action of a coagulant being broken up, and without a loss of dewatering capability.
Because the filtering fluid is separated from the outer tube and a cake layer that is captured in the minute holes of the screen is continuously scraped away by the scraper, clogging of the screen is prevented over the entire region, from the upstream region to the downstream region of the condensing zone, so that even in the case of low-concentration sludge having a high water load, it is possible to discharge a large amount of filtered fluid from the screen of the outer tube of the condensing zone, without clogging thereof.
The screw vane winding around the screw shaft has a pitch in the condensing zone that is narrower than the screw vane pitch in the filtering and dewatering zone, and if the outer tube is caused to rotate so that a balance is maintained in the supply of sludge from the condensing zone to the filtering and dewatering zone, the number of times the scraper comes into sliding contact with the screw is increased, filtered fluid being separated from the screen, in which clogging is prevented, enabling movement of a high-concentration sludge to the filtering and dewatering zone, thereby increasing the sludge processing capacity.
If a screw vane having a single pitch is wound around the screw shaft and a screw vane is multiply wound around the screw shaft in the condensing zone, the number of scraper contacts with the screen increases in proportion to the number of screw vanes, thereby restoring the screen surface.
If the outer tube of the condensing zone is caused to rotate in a direction opposite from the screw shaft, the relative rotational speed of the condensing zone with respect to the screw shaft rotational speed is increased, thereby increasing the number of times that the scraper provided on the screw vane makes sliding contact with the outer tube screen, so as to prevent clogging of the screen.
If screw vanes are multiply wound around the screw shaft of the condensing zone, the condensing zone outer tube and screw shaft are caused to rotate in one and the same direction, and the relative rotational speed with respect to the screw shaft of the outer tube of the condensing zone is made higher than the rotational speed of the screw shaft, the sludge is moved while being concentrated by the action of the rotation of the outer tube and the insertion pressure, and the number of times the scraper comes into sliding contact with the screen is increased.
By providing a plurality of screens on the outer tube in the filtering and dewatering zone and step-wise reducing the size of the minute holes in the screen tensioned on the condensing zone and a dewatering zone screen proceeding from upstream region to the downstream region, as the pressure applied to the sludge increases, only the filtered fluid is discharged, without the sludge flowing out from the minute holes, thereby improving the ability to reclaim the cake.
If the outer tube in the condensing zone and the filtering and dewatering zone is formed cylindrically, with a screw shaft provided therewithin having a diameter of which increased in a tapered manner, proceeding from the upstream region toward the downstream region, or if the diameter of the outer tube in the condensing zone and the filtering and dewatering zone is caused to be reduced, proceeding from the upstream region toward the downstream zone, a screw shaft being provided within this outer tube, the spacing between the outer tube and the screw shaft is relatively reduced in the direction of cake discharge, so that sludge from which filtered water is separated in the condensing zone is gradually dewatered in the outer tube in the filtering and dewatering zone, thereby enabling a high degree of dewatering of solids.
If the outer tube in the condensing zone and the filtering and dewatering zone is formed cylindrically, and a condensing zone cylindrical screw and filtering and dewatering zone screw shaft having a tapered diameter increasing from the upstream region towards the downstream region are linked within the outer tube, the filtering surface area of the condensing zone is increased, thereby achieving suitability for sludge having a low concentration.
If the outer tube in the condensing zone and filtering and dewatering zone is formed cylindrically, and a screw shaft is provided therein coaxial to the outer tube, use is also possible as a thickener, with an increased processing capacity and a reduced cost of fabrication.